Gas flow measurement devices are known which use a control valve as the throttling member to provide data for calculating flow rate on the basis of the degree of opening, the absolute inlet pressure and temperature, the total pressure drop across the valve and the physical properties of the gas. Compared with measurement members of fixed restriction type (orifice plates, nozzles etc.), calculating the flow rate with these known devices is more complicated. Accurate modelling of pressure dissipation and recovery remains particularly arduous because of the difficulty in locating the position of the vena contracta as the degree of opening and the discharge conditions (in terms of Reynolds number and Mach number) vary, making it impossible in the case of the most commonly used valves to measure the effective pressure in the contracted section.
The problem of obtaining a sufficiently accurate model of the pressure recovery downstream of the restricted section is rather complicated as it is always accompanied by turbulent dissipation. This results in pulsation in the pressure measurement downstream of the restriction and hence requires a downstream pressure tap to be located at a distance of 5-10 diameters from the valve. A consequence of this lack of accurate knowledge of the flow within the valve is the need for experimental tests to obtain the characteristics of a non-specialized valve for use as a measurement member.
In practice, the difficulties in modelling the expansion through the variable geometry restriction and the dissipation within the downstream ducts are solved experimentally by a large number of tests under different operating conditions.
On the other hand, there are numerous examples of fixed restriction measurement members which have been designed and experimented to give good repeatability of the shape and location of the vena contracta and to obtain an isentropic flow upstream of the restriction, thus enabling the flow rate to be calculated by very accurate formulas. Good accuracy of these members is however limited to relatively narrow operating ranges in terms of flow rate and pressure drop. These limits, which can be accepted in the practical application of fixed devices, become a problem in the case of valves which not only have to control flow but also have to simultaneously operate under the most extensive discharge conditions, from subsonic regimes to critical conditions.